1. Field of the Invention
This invention relates to the coating of particles with a continuous, dense, protective layer. The invention is particularly applicable to the encapsulation of organic and inorganic pigments to impart improved chemical, thermal and light stability thereto.
2. Description of the Prior Art
Much work has been done on the encapsulation or coating of inorganic and organic pigments with silica or a combination of silica and alumina as exemplified by a number of patents such as U.S. Pat. No. 3,370,971. The particles are encapsulated primarily to improve one or more of the properties of the pigment such as heat or chemical stability. However, the effectiveness of these coatings is somewhat limited for a number of reasons. For example, they tend to be brittle. Furthermore, their use is restricted, for the most part, to inorganic pigments. The procedure for applying the coatings is somewhat complicated and the heat stability of the coated pigments is only marginally acceptable.
A method of coating pigment particles of TiO.sub.2 with a layer of aluminum orthophosphate is described in U.S. Pat. No. 2,668,776. This method involves the addition of an alkali metal orthophosphate to a suspension of the pigment followed by the addition of aluminum salt to the suspension to cause mass precipitation of a gelatinous aluminum orthophosphate onto the pigment particles. The porous coating represents between 1 and 3.5% of the total weight of the partially coated particle and serves to improve the drying rate and stability of the pigment in a paint film.
The color fastness and weather stability of lead chromates are improved according to the teachings of U.S. Pat. No. 3,434,857, by coating the same with between 4 and 8% by pigment weight of a mixture of sodium silicate and antimony trifluoride or other compound capable of yielding ions of antimony and fluoride. The favorable results are achieved only by following the precise sequence of addition and by maintaining close control of the pH.
German DAS No. 1,288,714 describes a method of coating lead chromate pigment particles with a layer of zinc phosphate for improved H.sub.2 S resistance. Zinc in the form of zinc oxide is reacted with phosphoric acid at a pH of 6 or more to form the coating. The zinc is known to form a simple salt solution with the phosphate and does not normally form a complex with the phosphate ion.